Mode of action of CDK4/6 inhibitors. Notes: Palbociclib, ribociclib and abemaciclib inhibit the complex CDK4/cyclin D1, responsible for the phosphorylation and inhibition of the Rb tumor-suppressor gene's product. When Rb is phosphorylated, the cell proceeds from G1 to Phase S of the cell cycle, replicating DNA and preparing for mitosis.